The invention is directed to the length and/or speed measurement of a relatively moving surface and in particular to measurement apparatus in which a signal from an interference pattern on the surface is detected and corrected.
Presently used length and/or speed measuring devices for solid materials are of the contact type wherein a roller or other such device is in contact with the surface. This roller feeds a mechanical or electronic counter which will indicate either the length of the surface which has passed the device or the instantaneous speed of the surface as it moves relative to the device. These devices, though in widespread use, are found wanting in that they may be up to 3% or more in error due to slippage, wear, or other mechanical failure. In addition, they may be unable to operate in some hostile environments, such as with surfaces at high temperatures, or with materials that would be easily damaged by a contacting sensor.
Non-contact speed measurement techniques using the laser Doppler method, have been made to measure the speed of liquid or gas flow. These are known as Laser Doppler Velocimeters (LDV) and are described in the publications by J. B. Abbiss, T. W. Chubb and E. R. Pike, "Laser Doppler Anemometry," Optics and Laser Technology, December 1974, pp 249-261; and by C. Greated and T. S. Durrani, "Signal Analysis for Laser Velocimeter Measurements," Journal of Physcis E: Scientific Instruments, 1971, Vol. 4, pp 24-26. This type of velocimeter suffers a drop-out problem whereby the Doppler signal at times assumes very low amplitudes and cannot be simply detected. Present methods of overcoming the drop-out problem to achieve acceptable precision are generally very expensive.